Method for measuring trace quantity of oxygen in gas

ABSTRACT

The present invention relates to a method and an apparatus for measuring trace quantity of oxygen in a gas by reacting yellow phosphorus vapor and oxygen in a sample gas, and measuring the intensity of the light emitted by the reaction. Together with the sample gas, a constant amount of oxygen is continuously supplied to a reaction chamber so as to react with the yellow phosphorus vapor, and the intensity of the light emitted is measured. The oxygen concentration in the sample gas is determined from the difference between the value measured as above and the intensity of the light emitted by the reaction between the yellow phosphorus vapor and the added oxygen, or from the oxygen concentration obtained from the measured value and the concentration of the added oxygen. The constant amount of oxygen to be added is supplied by supplying a constant amount of a preliminarily prepared oxygen-containing gas together with the sample gas or by providing an oxygen permeating membrane in the yellow phosphorus vapor supplying system so as to supply the oxygen by the permeation of the oxygen in atmosphere. The small amount of oxygen with a concentration of several ppb in the sample gas can be continuously measured with high precision.

TECHNICAL FIELD

The present invention relates to a method and an apparatus for measuringtrace quantity of oxygen in sample gas. More particularly, the presentinvention relates to a method and an apparatus for measuring tracequantity of oxygen in gas employing luminous reaction between the tracequantity of oxygen in the sample gas and yellow phosphorus vapor.

BACKGROUND ART

For measuring the concentration of trace quantity of oxygen in anindustrial gas such as nitrogen, hydrogen, argon or helium, the luminousreaction between oxygen and yellow phosphorus vapor is employed. Theintensity of light generated by the reaction is measured by aphotodetector such as a photomultiplier. The yellow phosphorus vapor isconventionally obtained by the sublimation of solid yellow phosphorus atroom temperature (15°-25° C.).

A method and an apparatus for measuring trace quantity of oxygen in agas using the yellow phosphorus vapor are described in, for example,Japanese Laid Open Patent Application (Kokai) No. 63-302348. These willnow be described referring to FIGS. 7 and 8.

An apparatus for measuring trace quantity of oxygen in a gas shown inFIG. 7 comprises a reaction chamber 10, a sample gas supplying duct 11for supplying the sample gas to the reaction chamber 10, a yellowphosphorus vapor supplying duct 13 for supplying the yellow phosphorusvapor to the reaction chamber 10, which is generated from solidphosphorus P contained in a yellow phosphorus container 12, aphotodetector 14 such as a photomultiplier for measuring the intensityof the light generated by the reaction between the oxygen in the samplegas and the yellow phosphorus vapor in the sample gas in the reactionchamber 10.

The quantity of oxygen in the sample gas is detected by supplying thesample gas through the sample gas supplying pipe 11 to the reactionchamber 10 and simultaneously supplying yellow phosphorus vaporgenerated from the solid yellow phosphorus P to the reaction chamber 10through the yellow phosphorus vapor supplying duct 13 so as to allow areaction between the yellow phosphorus vapor and the oxygen in thesample gas, followed by measuring the intensity of the light generatedby the reaction by a photodetector 14 so as to determine the oxygenconcentration based on the output from the photodetector.

An apparatus for measuring trace quantity of oxygen in a gas shown inFIG. 8 comprises a reaction chamber 15, a sample gas supplying pipe 11for supplying the sample gas to the reaction chamber 15, a container 16provided in the reaction chamber 15 for harboring solid yellowphosphorus P, a photodetector 14 such as a photomultiplier for measuringthe intensity of light emitted by the reaction between the yellowphosphorus vapor and the oxygen in the sample gas.

The quantity of oxygen in the sample gas is measured by reacting theyellow phosphorus vapor sublimated in the reaction chamber 15 and theoxygen in the sample gas so as to emit light, and by measuring theintensity of the light emitted by the reaction.

However, in the application fields of industrial gases, for example, inthe field of semiconductor, it is demanded to accurately determine theoxygen level in a gas to be used in the order of ppb.

Although the trace quantity of oxygen may be determined in the order of1 ppm or less by the above-described conventional method and apparatusby controlling the amount of the yellow phosphorus, it is difficult todetect the extremely small amount of oxygen with a level of several ppbor less with high precision by the conventional method and apparatus.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method and anapparatus for measuring extremely small amount of oxygen contained in asample gas at a level of several ppb or less with high precision.

Another object of the present invention is to provide a method and anapparatus for measuring trace quantity of oxygen contained in a gas bymeasuring the quantity of oxygen after adding a constant amount ofoxygen into the sample gas, thereby measuring the quantity of oxygen inthe sample gas before the addition of oxygen at a level of several tensppb or less, particularly 30 ppb or less or even several ppb or lesswith high precision.

Still another object of the present invention is to provide a method andapparatus for measuring trace quantity of oxygen in a gas which excelsin reproducibility and stability, which make it possible to continuouslydetermining oxygen in a number of sample gases by continuously adding aconstant amount of oxygen.

Still another object of the present invention is to provide a method andapparatus for measuring trace quantity of oxygen in a gas in which aconstant amount of oxygen can be provided by supplying the oxygen to thesample gas through an oxygen permeating membrane so that there is noneed to separately provide oxygen to be added, by which oxygen may bemeasured stably and continuously by an apparatus with simple structure.

According to the first aspect of the present invention, there isprovided a method for measuring trace quantity of oxygen in a gas byreacting yellow phosphorus vapor with oxygen in a sample gas andmeasuring the quantity of the oxygen contained in the sample gas basedon the intensity of light emitted by the reaction, characterized in thata constant amount of adding oxygen is continuously supplied togetherwith the sample gas and the intensity of the light emitted by thereaction between the oxygen and yellow phosphorus is measured.

According to the second aspect of the present invention, there isprovided a method for measuring trace quantity of oxygen in a gascharacterized in that the oxygen level is determined from the differencebetween the measured value obtained by the method according to the firstaspect of the present invention and the light intensity emitted by thereaction between the yellow phosphorus vapor and the added oxygen.

According to the third aspect of the present invention, there isprovided a method for measuring trace quantity of oxygen in a gascharacterized in that oxygen concentration corresponding to the measuredvalue obtained by the method according to the first aspect of thepresent invention is determined, and the oxygen concentration in thesample gas is calculated by subtracting the concentration of the addedoxygen from the thus determined oxygen concentration.

According to the fourth aspect of the present invention, there isprovided an apparatus for measuring trace quantity of oxygen in a gas,comprising a reaction chamber in which yellow phosphorus vapor andoxygen in a sample gas are reacted, and photodetecting means formeasuring intensity of light emitted by the reaction, characterized inthat the apparatus further comprises a sample gas supplying pipe forsupplying the sample gas to the reaction chamber, a yellow phosphorusvapor supplying pipe for supplying yellow phosphorus vapor to thereaction chamber, and an oxygen supplying pipe for supplying anoxygen-containing gas to the sample gas supplying pipe or to thereaction chamber.

According to the fifth aspect of the present invention, there isprovided an apparatus for measuring trace quantity of oxygencharacterized by comprising an oxygen permeating membrane in the oxygensupplying pipe according to the fourth aspect of the present invention.

According to the sixth aspect of the present invention, there isprovided an apparatus for measuring trace quantity of oxygen in a gas,comprising a reaction chamber in which yellow phosphorus vapor andoxygen in a sample gas are reacted, and photodetecting means formeasuring intensity of light emitted by the reaction, characterized inthat the apparatus further comprises a sample gas supplying pipe forsupplying the sample gas to the reaction chamber, a yellow phosphorusvapor supplying pipe for supplying yellow phosphorus vapor to thereaction chamber, and an oxygen permeating membrane provided in thesample gas supplying pipe or in the yellow phosphorus supplying pipe.

According to the seventh aspect of the present invention, there isprovided an apparatus for measuring trace quantity of oxygen in a gas,characterized in that the oxygen permeating membrane employed in theapparatus according to the fifth or sixth aspect of the presentinvention is placed in a thermostatic bath.

By adding a constant amount of oxygen, the oxygen concentrationsubjected to the reaction can be controlled to the optimum range in thecalibration curve of the photomeasuring means such as a photodetector,so that extremely small amount of oxygen may be measured with highprecision. By adding the oxygen through an oxygen permeating membrane, aconstant amount of oxygen may readily be added. Further, by controllingthe temperature of the environment of the oxygen permeating membrane byplacing the membrane in a thermostatic bath, the amount of the oxygen tobe added can be controlled.

BRIEF DESCRIPTION OF THE INVENTION

FIGS. 1-3 show embodiments of the present invention.

FIG. 1 is a system diagram showing an embodiment in which a constantamount of oxygen-containing gas is added to the sample gas or to thereaction chamber.

FIG. 2 is a system diagram showing an embodiment in which a constantamount of oxygen is supplied through an oxygen permeating membrane.

FIG. 3 is a system diagram showing another embodiment in which aconstant amount of oxygen is supplied through an oxygen permeatingmembrane.

FIG. 4 shows an example of a calibration curve prepared by using astandard gas containing a known concentration of oxygen and anoxygen-containing gas to which a constant amount of oxygen is addedemploying the apparatus shown in FIG. 1.

FIG. 5 is an example of a calibration curve prepared by using a standardgas containing a known concentration of oxygen employing the apparatusshown in FIG. 1.

FIG. 6 is an example of a calibration curve prepared by using a standardgas containing a known concentration of oxygen employing the apparatusshown in FIG. 2.

FIGS. 7 and 8 are system diagrams each showing a conventional apparatusfor measuring trace amount of oxygen.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, oxygen in a sample gas and a constant amountof oxygen continuously added to the sample gas react with yellowphosphorus vapor in a reaction chamber. Oxygen spontaneously reacts withyellow phosphorus upon contact therewith to emit light. Since theintensity of the light emitted by the reaction is proportional to theamount of the oxygen supplied to the reaction chamber, the amount of theoxygen can be determined by measuring the intensity of the light. Themeasurement of the intensity of the light may be carried out by usingconventional photodetecting means such as a photomultiplier.

Controlled amount of the sample gas is supplied to the reaction chamberthrough a sample gas supplying pipe.

The yellow phosphorus vapor may be obtained by sublimation of solidyellow phosphorus at room temperature (15°-25° C.). The amount of theyellow phosphorus vapor supplied to the reaction chamber may becontrolled by controlling the flow rate of a carrier gas for yellowphosphorus vapor and/or the temperature of the solid phosphorus. Thecarrier gas for yellow phosphorus vapor may preferably be inert toyellow phosphorus and must not substantially contain oxygen. Examples ofthe carrier gas for yellow phosphorus vapor include inert gases such asnitrogen, argon and helium, and hydrogen. If oxygen is contained in thecarrier gas for yellow phosphorus vapor, the carrier gas should be usedafter deoxidation.

Means for continuously supplying a constant amount of oxygen to thesample gas is largely classified into two groups.

The first means is for supplying a controlled amount of pure oxygen oran oxygen-containing gas containing a constant amount of oxygen to thereaction chamber through an oxygen supplying pipe and the sample gassupplying pipe or directly to the reaction chamber through an oxygensupplying pipe. The oxygen-containing gas may preferably be a mixed gasof a constant amount of oxygen and a gas which is inert to yellowphosphorus vapor.

The second means is for supplying a constant amount of oxygen to the gasin the sample gas supplying pipe or the yellow phosphorus vaporsupplying pipe by providing an oxygen permeating membrane in one ofthese pipes using the difference in the partial pressures of the oxygenin the pipe and the oxygen in atmosphere. The amount of the oxygen maybe controlled by the effective area of the oxygen permeating membrane,temperature of the atmosphere or the flow rate of the gas in the pipe.In this case, a carrier gas not substantially containing oxygen isintroduced into the oxygen supplying pipe. If oxygen is supplied to theyellow phosphorus vapor supplying pipe through the oxygen permeatingmembrane, although the luminous reaction initiates in the yellowphosphorus vapor supplying pipe, since the reaction rate is considerablyslow, measurement can be properly carried out if an amount of oxygensufficient for continuing the luminous reaction in the reaction chamberis introduced into the yellow phosphorus vapor supplying pipe.

The intensity of the light emitted in the reaction chamber is measuredby photodetecting means.

The oxygen concentration in the sample gas may be determined from thedifference between the thus obtained measured value and the intensity oflight emitted by the reaction between the yellow phosphorus vapor andthe added oxygen. Alternatively, the oxygen concentration correspondingto the measured value is determined and the oxygen concentration in thesample gas may be calculated by subtracting the concentration of theadded oxygen from the thus determined oxygen concentration. Moreconcretely, the oxygen concentration may be determined by comparing themeasured value with a calibration curve as described later.

The present invention will now be described in more detail based on theembodiments shown in the drawings.

FIG. 1 is a system diagram showing the first embodiment of the apparatusfor measuring trace quantity of oxygen in a gas according to the presentinvention.

The apparatus for measuring trace quantity of oxygen comprises areaction chamber 20, a sample gas supplying system 21 for supplying thesample gas to the reaction chamber 20, a yellow phosphorus vaporsupplying system 22 for supplying yellow phosphorus vapor to thereaction chamber 20, an oxygen supplying system 23 for supplying aconstant amount of oxygen to the reaction chamber 20 and aphotodetecting means 24 such as a photomultiplier for measuring theintensity of the emitted light.

The reaction chamber 20 and a measurement chamber 25 accommodating thephotodetector 24 constitute a measurement unit 27 such that a singlechamber is separated into the adjacent two chambers by a lighttransmitting plate 26 such as an optical glass. The reaction chamber 20is air-tight and an exhaust outlet 20a for exhausting the gas after thereaction is provided therein.

The sample gas supplying system 21 comprises a sample gas supplying pipe29 equipped with a sample gas flow meter 28 which can control the flowrate of the sample gas.

The yellow phosphorus supplying system 22 comprises a container 31 forharboring solid yellow phosphorus, which is placed in a thermostaticbath 30, a carrier gas supplying pipe 32 for supplying a carrier gaswhich carries yellow phosphorus vapor obtained by the sublimation of thesolid phosphorus P in the container 31, a yellow phosphorus vaporsupplying pipe 33 for supplying the carrier gas carrying the yellowphosphorus vapor to the reaction chamber 20, a carrier gas flow meter 34provided in the carrier gas supplying pipe 32, which can control theflow rate of the carrier gas, and a bypass 35 connected to the carriergas supplying pipe 32 and the yellow phosphorus vapor supplying pipe 33,which bypasses the container 31 containing the solid yellow phosphorus.The bypass 35 comprises valves 36a and 36b respectively providedupstream and downstream the container 31 containing the solid yellowphosphorus, and a bypass valve 36c provided in a bypass pipe 37. Thebypass 35 can supply only the carrier gas to the reaction chamber 20.The carrier gas for the yellow phosphorus vapor is preferably inert toyellow phosphorus and must not substantially contain oxygen. Examples ofthe carrier gas for yellow phosphorus vapor include inert gases such asnitrogen, argon and helium, as well as hydrogen. If the carrier gascontains oxygen, the carrier gas is used after deoxidation.

The oxygen supplying system 23 comprises an oxygen supplying pipe 39equipped with a flow meter 38 which can control the flow rate. Thedownstream portion of the oxygen supplying pipe 39 is connected to thesample gas supplying pipe 29 as shown in FIG. 1 by the solid line orconnected directly to the reaction chamber 20 as shown in FIG. 1 by thephantom line. An oxygen-containing gas is used for supplying a constantamount of oxygen from the oxygen supplying system 23 to the reactionchamber through the sample gas supplying pipe 29 or directly to thereaction chamber. As the oxygen-containing gas, although pure oxygen maybe employed, it is preferred to employ a mixed gas of a gas inert to theyellow phosphorus vapor and a constant amount of oxygen.

The operations for measuring trace amount of oxygen using theabove-described apparatus will now be described.

The valves 36a and 36b in the bypass 35 are closed and the bypass valve36c is opened, and the carrier gas for yellow phosphorus vapor issupplied to the reaction chamber 20 through the carrier gas supplyingpipe 32, bypass pipe 37 and yellow phosphorus vapor supplying pipe 33 soas to purge the yellow phosphorus vapor supplying pipe 33 and thereaction chamber 20. The sample gas supplying pipe 29 and the oxygensupplying pipe 39 are also purged with a gas not containing oxygen, forexample, a gas after passing through a deoxidizer.

A blank test is then carried out. That is, a gas not containing oxygen(blank gas), for example, a gas after passing through a deoxidizer, issupplied to the reaction chamber 20 from the sample gas supplying pipe29 at a prescribed flow rate by controlling the flow rate by the samplegas flow meter 28. Simultaneously, an oxygen-containing gas (e.g., a gasprepared by adding a prescribed amount of oxygen gas to nitrogen gas sothat the oxygen concentration therein is controlled) is supplied to thereaction chamber 20 from the oxygen supplying duct 39 through the samplegas supplying duct 29 or directly to the reaction chamber from theoxygen supplying pipe 39. Then the valves 36a and 36b in the bypass 35are opened and the bypass valve 36c is closed. The carrier gas foryellow phosphorus vapor of which flow rate is controlled by the carriergas flow meter 34 is supplied to the container 31 containing solidyellow phosphorus, which container is preliminarily kept at a prescribedtemperature by the thermostatic bath 30, and the yellow phosphorusgenerated by the sublimation of the solid yellow phosphorus P issupplied to the reaction chamber 20 through the yellow phosphorus vaporsupplying pipe 33. The yellow phosphorus vapor reacts with the oxygencontained in the oxygen-containing gas supplied to the reaction chamber20 and a light is emitted thereby. The intensity of the emitted light ismeasured by the photodetector 24 and a photoelectric currentcorresponding to the amount of the added oxygen is outputted. Since thethus obtained output value is the output value obtained when the oxygenconcentration in the sample gas is zero, zero-adjustment is carried outby setting this output value to the zero point of the oxygenconcentration in the sample gas.

After conducting the zero-adjustment by the blank test as describedabove, the oxygen concentration in a sample gas is carried out. That is,the sample gas is supplied from the sample gas supplying pipe to thereaction chamber after controlling the flow rate to a prescribedconcentration by the sample gas flow meter 28. At this time, theoxygen-containing gas and the yellow phosphorus vapor are continuouslysupplied to the reaction chamber 20. The oxygen in the sample gas andthe oxygen-containing gas supplied to the reaction chamber 20 react withyellow phosphorus vapor in the reaction chamber, thereby emitting alight. The intensity of the emitted light is measured by thephotodetector 24 and a photoelectric current is outputted.

The output value is compared with a preliminarily prepared calibrationcurve and the oxygen concentration in the sample gas may be obtained asa difference between the output value obtained when measuring the oxygenconcentration in the sample gas and the output value in the blank test.

The measurement of the trace quantity of oxygen will now be described byway of examples.

Firstly, a calibration curve covering the expected range of the oxygenlevel in the sample gas is provided.

More particularly, standard gases containing oxygen in the concentrationof 10 ppb, 20 ppb, 30 ppb, 40 ppb, 50 ppb, 60 ppb and 70 ppb,respectively, and an oxygen-containing gas with an oxygen concentrationof 360 ppb are provided. Firstly, nitrogen gas after passing through adeoxidizer (blank gas) is supplied to the reaction chamber through thesample gas supplying system 21 at a flow rate of 800 ml/min, nitrogengas for carrying yellow phosphorus vapor is supplied to the reactionchamber 20 through the yellow phosphorus supplying system 22 at a flowrate of 300 ml/min, and an oxygen-containing gas with an oxygenconcentration of 360 ppb is supplied to the reaction chamber 20 throughthe oxygen supplying system 23 at a flow rate of 100 ml/min so as toattain an oxygen concentration in the reaction chamber 20 of 30 ppb. Theintensity of the emitted light in the reaction chamber 20 at this pointis measured by the photodetector 24 and zero-adjustment is carried out.

A standard gas with an oxygen concentration of 10 ppb is then suppliedto the reaction chamber through the sample gas supplying system 21 inplace of the blank gas. At this time, the nitrogen gas carrying theyellow phosphorus vapor and the oxygen-containing gas are continuouslysupplied to the reaction chamber 20. The intensity of the light emittedfrom the reaction chamber 20 is measured by the photodetector 24 and theobtained output value is plotted. In the same manner, the output valuesobtained by using the standard gases with oxygen levels of 20 ppb-70 ppbare plotted. By connecting the plotted points with a line, a calibrationcurve A shown in FIG. 4 corresponding to the oxygen concentrations of 0ppb to 70 ppb is obtained. Thus, in the calibration curve A, the outputobtained when the oxygen level in the reaction chamber 20 is 30 ppbcorresponds to the oxygen concentration of 0 ppb in the sample gas. InFIG. 4, the abscissa indicates the oxygen concentration in the standardgas, and the ordinate indicates the difference in terms of percentbetween the output obtained by using the standard gas and the outputemployed for the zero-adjustment.

It should be noted that since the oxygen contained in the standard gasis diluted and mixed with the nitrogen gas carrying the yellowphosphorus vapor and the oxygen-containing gas in the reaction chamber20, for example, when the standard gas with an oxygen concentration of10 ppb is supplied to a reaction chamber 20, the oxygen concentration inthe reaction chamber 20 is 36.7 ppb.

After preparing the calibration curve A, measurement of the oxygenconcentration in the sample gas is carried out. That is, the nitrogengas carrying the yellow phosphorus vapor and the oxygen-containing gasare supplied to the reaction chamber in the manner described above, andsimultaneously a sample gas with an oxygen level of x is supplied to thereaction chamber through the sample gas supplying system 21 at a flowrate of 800 ml/min. The output from the photodetector 24 is measured andthe oxygen concentration is determined based on the calibration curve A.For example, if the output difference is 10%, the oxygen concentrationin the sample gas is determined to 10 ppb from the calibration curve A.

Since the oxygen in the sample gas is diluted and mixed with thenitrogen gas carrying the yellow phosphorus vapor and with theoxygen-containing gas in the reaction chamber, for example, if a samplegas with an oxygen concentration of 10 ppb is supplied to the reactionchamber 20, the oxygen concentration in the reaction chamber 20 is 36.7ppb. However, since the zero-adjustment is carried out when the oxygenlevel in the reaction chamber 20 is 30 ppb, and since the oxygen in thesample gas is diluted under the same conditions as the oxygen in thestandard gases when preparing the calibration curve, the oxygenconcentration in the sample gas can directly be determined from theobtained output difference.

Next, the procedures for measuring the oxygen level in the sample gasfrom the calibration curve based on the oxygen concentration in thereaction chamber 20 will now be described referring to FIG. 5. Acalibration curve B covering the oxygen levels of 0 ppb to 100 ppb isprepared by connecting the plotted output values obtained by introducingnitrogen gas not containing oxygen into the oxygen supplying pipe 23 ata flow rate of 100 ml/min in place of the oxygen-containing gas with anoxygen concentration of 360 ppb, introducing the other gases asdescribed above so as to carry out the zero-adjustment, supplyingstandard gases with known oxygen concentrations to the reaction chamber20 through the sample gas supplying pipe 21 so as to change the oxygenconcentration in the reaction chamber 20 to 10 ppb, 20 ppb, 30 ppb, 40ppb, 50 ppb, 60 ppb, 70 ppb, 80 ppb, 90 ppb and 100 ppb, and plottingthe output values. In FIG. 5, the abscissa indicates the oxygenconcentration in the reaction chamber 20 and the ordinate indicates theoutput value in terms of percent. Thus, in the calibration curve B, thezero point in the calibration curve A, that is, the oxygen concentrationin the reaction chamber 20 of 30 ppb is indicated as 16%.

The oxygen concentration in the sample gas may be determined as followsusing the calibration curve B.

I. Measurement of Reference Value (Zero-adjusting value)

(a) To Sample Gas Supplying System 21

Nitrogen gas after passing through a deoxidizer at a flow rate of 800ml/min.

(b) To Yellow Phosphorus Supplying System 22

Carrier gas for yellow phosphorus vapor (nitrogen gas) at a flow rate of300 ml/min.

(c) To Oxygen-supplying System 23

Nitrogen gas containing 360 ppb of oxygen at a flow rate of 100 ml/min.

Therefore, the oxygen concentration in the reaction chamber is 30 ppb,and the output of the photodetector at this point is 16% as shown inFIG. 5. This output is defined as the output of the sample gas with anoxygen level of zero.

II. Measurement of Oxygen Level in Sample Gas

(a) To Sample Gas Supplying System 21

Sample gas with an oxygen concentration of x ppb at a flow rate of 800ml/min.

(b) To Yellow Phosphorus Vapor Supplying System 22

Carrier gas (nitrogen gas) for yellow phosphorus vapor at a flow rate of300 ml/min.

(c) To Oxygen Supplying System 23

Nitrogen gas containing 360 ppb of oxygen in nitrogen at a flow rate of100 ml/min.

The oxygen concentration c ppb in the reaction chamber 20 is representedby the formula: ##EQU1##

If the output value from the photodetector at this point is 20%, theoxygen concentration in the reaction chamber 20 corresponding to theoutput value of 20% is 34.5 ppb as determined from the calibration curveB in FIG. 5.

By substituting c in the above equation for 34.5, an equation of##EQU2## is obtained.

By solving this equation, x is 6.75, so that the oxygen level x to bedetermined in the sample gas is 6.75 ppb.

In cases where measurement is carried out according to theabove-described procedures, memory function and processing function areprovided in the analyzer (the measuring apparatus of the presentinvention), and the calibration curve and the calculation are soconstituted as to automatically calculate the measured values and todisplay them by means of these functions. In this case, the oxygenconcentration in the sample gas may be determined from the outputdifference of the photodetector or by measuring the oxygen concentrationafter addition of oxygen and subtracting the added oxygen concentrationfrom the measured oxygen concentration.

Thus, the oxygen concentration in the sample gas may also be determinedby determining the total concentration of the oxygen supplied to thereaction chamber 20 from the sample gas supplying system 21 and theoxygen supplying system 23, and calculating the difference between thethus measured oxygen concentration and the concentration of the addedoxygen.

As is apparent from the calibration curve shown in FIG. 5, by making theoxygen concentration in the reaction chamber 20 to not less than 30 ppbby continuously adding oxygen with a concentration of 30 ppb to thereaction chamber 20 as mentioned above, even if the oxygen concentrationin the sample gas is about 5 ppb, output difference of about 5% isobtained, so that the measurement of the oxygen concentration in thesample gas can be sufficiently carried out. On the other hand, if nooxygen is added to the reaction chamber 20, even if the oxygen level inthe sample gas is about 10 ppb, output difference of about only 2-3% isobtained, which cannot be distinguished from noise of the photodetector24. Thus, in the calibration curve used in the present invention, theoutput difference corresponding to the oxygen level in the sample gas ofnot more than 20 ppb is made large, so that extremely small amount ofoxygen may be measured with high precision.

Thus, by continuously adding oxygen with a concentration of, forexample, 30 ppb to the reaction chamber 20, the oxygen level measured bythe measurement unit 27 may be adjusted to the range optimum for thephotodetector 24, so that the trace quantity of oxygen in the sample gasat a concentration of several ppb may be determined accurately with highprecision.

FIGS. 2 and 3 shows different embodiments in which oxygen to be added tothe sample gas is obtained by an oxygen permeating membrane. It shouldbe noted that the same reference numerals are used for denoting the sameelements as in FIG. 1 and the explanations thereof are omitted here.

The apparatus for measuring trace quantity of oxygen shown in FIG. 2comprises a reaction chamber 20, a sample gas supplying system 21, ayellow phosphorus vapor supplying system 22 and a photodetector 24 suchas a photomultiplier for measuring the light intensity, as in theapparatus shown in FIG. 1, but the oxygen supplying system 23 shown inFIG. 1 is not provided.

An oxygen introducing unit 40 is provided in the sample gas supplyingpipe 29 in the sample gas supplying system 21 or in the yellowphosphorus vapor supplying pipe 33 in the yellow phosphorus vaporsupplying system 22.

The oxygen introducing unit 40 comprises a tubular oxygen permeatingmembrane 41 and a thermostatic bath 42 for keeping the temperature ofthe environment of the oxygen permeating membrane 41 at a prescribedtemperature, which contains therein the oxygen permeating membrane 41.The oxygen permeating membrane 41 is prepared by forming a materialwhich permeates oxygen into tubes. Examples of the material whichpermeates oxygen include synthetic resins such as Teflon (trade name),ceramics such as zirconia, various rubbers, carbon-based membranes andion pumps.

By providing the oxygen introducing unit 40 in the sample gas supplyingpipe 29 or in the yellow phosphorus vapor supplying pipe 33, a constantamount of oxygen may be introduced into the sample gas or into thecarrier gas for yellow phosphorus vapor by exploiting the differencebetween the partial oxygen pressure in the oxygen permeating membrane 41and the partial oxygen pressure in atmosphere.

The environment in the thermostatic bath 42, that is, the environment ofthe oxygen permeating membrane 41 may be atmosphere air and the amountof the oxygen to be introduced may be controlled by controlling thesurface area of the oxygen permeating membrane 41, the temperature ofthe atmosphere thereof, flow rate or pressure of the gas in the pipe.

It should be noted that in cases where the oxygen introducing unit 40 isprovided in the yellow phosphorus vapor supplying pipe 33, althoughluminescence initiates at the downstream portion of the oxygenintroducing unit 40 by the reaction between the oxygen and the yellowphosphorus, since the reaction rate is considerably slow, by introducingoxygen in an amount sufficient to continue the reaction in the reactionchamber 20, the same effect obtained by introducing oxygen by anothermeans can be obtained.

By merely providing an oxygen permeating membrane 41 made of Teflon orthe like in the pipe for supplying the gas into the reaction chamber 20and placing the same in the thermostatic bath 42, a constant amount ofoxygen with a concentration of as small as several tens ppb may be addedwith such a very simple structure. Further, throughout the operation ofthe apparatus for measuring trace quantity of oxygen, oxygen may readilybe provided stably and continuously without the need for separatelyproviding oxygen to be added as in the apparatus shown in FIG. 1, sothat the measurement may be carried out continuously for long period oftime at a lower cost.

The measurement of trace quantity of oxygen will now be describedconcretely based on experimental examples.

A tubular oxygen permeating membrane 41 made of Teflon was provided inthe sample gas supplying pipe 29 and was placed in the thermostatic bath42. The Teflon tube is commercially available and had an inner diameterof 1.59 mm, outer diameter of 3.17 mm and a length of 20 mm. To make theamount of oxygen added to the sample gas through the oxygen permeatingmembrane 41 constant, it is necessary to make the supplying pressure ofthe sample gas and the flow rate constant and to place the oxygenpermeating membrane 41 in the thermostatic bath 42 so as to make thetemperature of the gas in and out of the oxygen permeating membrane 41constant. In this experimental example, the outer environment of theoxygen permeating membrane 41 was atmosphere and the temperature in thethermostatic bath 42 was set to 50° C.

Although the pressure in the tube of the oxygen permeating membrane issomewhat higher than the pressure outside the membrane (by about thepressure loss in the duct), since the oxygen concentration is higheroutside the tube than inside the tube, very small amount of oxygen inatmosphere permeates the membrane due to the difference in this partialoxygen pressures and is mixed with the sample gas flowing the pipe. Inthis case, in order to make the amount of the oxygen added to the samplegas through the oxygen permeating membrane 41 constant, it is necessaryto make the supplying pressure and the flow rate of the sample gasconstant, and to make the temperature of the oxygen permeating membrane41 and of the gas inside and outside the pipe constant. In thisexperimental example, the parameters are set such that 30 ppb of oxygenis continuously supplied to the sample gas when the sample gas issupplied at a flow rate of 800 ml/min.

As in the above-described experimental example, standard gases withoxygen concentrations of 10 ppb, 20 ppb, 30 ppb, 40 ppb, 50 ppb, 60 ppband 70 ppb, respectively, were supplied to the reaction chamber 20through the sample gas supplying pipe 29 at a flow rate of 800 ml/min,and nitrogen gas carrying a prescribed amount of yellow phosphorus vaporwas simultaneously supplied to the reaction chamber 20 through theyellow phosphorus vapor supplying pipe 29 at a flow rate of 300 ml/min.The output values obtained by using the standard gases were plotted andthe plotted points were connected with a line, which is a calibrationcurve C shown in FIG. 6.

Since the calibration curve C was prepared in the conditions in which 30ppb of oxygen was continuously admixed with the standard gases throughthe oxygen permeating membrane 41, this calibration curve corresponds tothe range of 30 ppb-100 ppb or the output value of 16%-100% of thecalibration curve prepared by using a gas in which oxygen is notadmixed. That is, if a standard gas containing 10 ppb of oxygen issupplied at a flow rate of 800 ml/min, the reaction chamber 20 is insuch a condition that a gas containing 40 ppb of oxygen is supplied tothe reaction chamber at a flow rate of 800 ml/min.

After preparing the calibration curve C as mentioned above, themeasurement of the oxygen concentration in the sample gas is carriedout. That is, nitrogen gas carrying the yellow phosphorus vapor issupplied to the reaction 20 as described above, and a sample gas with anoxygen level of x is supplied to the reaction chamber 20 through thesample gas supplying system 21 at a flow rate of 800 ml/min. The outputof the photodetector 24 at this point is read and the oxygenconcentration is determined based on the calibration curve C. Forexample, if the output difference is 4%, the oxygen concentration in thesample gas is determined to be 4.5 ppb from the calibration curve C.Alternatively, using the calibration curve obtained by using a gas towhich oxygen is not admixed, the oxygen level of 34.5 ppb is determinedfrom the output value of 20% and then the oxygen concentration of 4.5ppb in the sample gas may be determined by subtracting the oxygenconcentration of the added oxygen of 30 ppb from the oxygenconcentration of 34.5 ppb.

With the apparatus in which the oxygen introducing unit is provided inthe yellow phosphorus vapor supplying system 22, the oxygenconcentration in the sample gas may be determined in the same manner.

The apparatus for measuring trace amount of oxygen shown in FIG. 3 hasthe similar constitution to the apparatus shown in FIG. 1, which furthercomprises an oxygen introducing unit 40 in the oxygen supplying pipe 39in the oxygen supplying system 23. It should be noted that a carrier gasfor oxygen which is an inert gas substantially not containing oxygen issupplied to the oxygen supplying pipe 39.

By providing the oxygen introducing unit 40 in the oxygen supplying pipe39, a constant amount of oxygen may be added to the carrier gas foroxygen. Therefore, as the apparatus shown in FIG. 1, a constant amountof oxygen may be continuously supplied to the reaction chamber 20 fromthe oxygen supplying pipe 39 directly or through the sample gassupplying pipe 29.

In cases where the oxygen introducing unit 40 is provided in the oxygensupplying pipe 39, by controlling the flow rate of the carrier gas foroxygen, the amount of the oxygen added through the oxygen permeatingmembrane 41 may be controlled, so that an optimum amount of oxygen maybe added depending on the oxygen level in the sample gas.

In carrying out the measurement of the trace quantity of oxygen in asample gas with this apparatus, the measurement may be carried out inthe same manner as with the apparatus shown in FIG. 1. That is, bysetting the flow rate of the carrier gas for oxygen to 100 ml/min and bysetting the amount of oxygen added through the oxygen permeatingmembrane 41 to 360 ppb, the trace quantity of oxygen in the sample gasmay be determined in the same manner as described above.

Although the invention was described in detail based on specificembodiments thereof, it is apparent for those skilled in the art thatvarious modifications may be made without departing from the spirit andscope of the present invention. For example, the apparatus may beprovided with a function for indicating the measured values in terms ofthe above-described output difference or with a recorder in which afunction for obtaining the difference in concentrations is preliminarilyincorporated, or the apparatus may be designed as an alarm. Therefore,the above description must not be interpreted in any restrictive way.

We claim:
 1. In a method for measuring a trace quantity of oxygen in agas comprising reacting yellow phosphorus vapor with the oxygen in asample of said gas to provide light and measuring the quantity of theoxygen in the sample of said gas based on the measured intensity of thelight emitted by the reaction, said measured intensity having anon-linear detector response in the range of 0-100 parts-per-billionwherein the improvement comprises continuously supplying a constantamount of oxygen together with the sample of the gas, the amount ofoxygen continuously supplied being effective to shift the intensity ofthe light emitted by the reaction to a range in which the non-lineardetector response is substantially removed wherein the measuredintensity is the light emitted by the reaction of the yellow phosphorusvapor and both the oxygen in the sample and the oxygen continuouslysupplied with the sample.
 2. The method for measuring a trace quantityof oxygen in a gas according to claim 1, wherein the light emitted bythe reaction between the the yellow phosphorus vapor and both the oxygencontinuously supplied and the oxygen in the sample is measured by aphotodetector and the oxygen concentration is determined from the outputof the photodetector.
 3. The method for measuring a trace quantity ofoxygen in a gas according to claim 1, wherein the constant amount ofoxygen is continuously supplied by supplying an oxygen-containing gascontaining the constant amount of oxygen.
 4. The method for measuring atrace quantity of oxygen in a gas according to claim 1, wherein theconstant amount of oxygen to be continuously supplied is supplied to thesample gas or to a carrier gas for yellow phosphorus vapor through anoxygen permeating membrane.
 5. The method for measuring a trace quantityof oxygen in a gas according to claim 4, wherein the amount of theconstant quantity of oxygen to be continuously supplied is controlled bycontrolling the temperature in a thermostatic bath harboring the oxygenpermeating membrane by using the difference in the partial pressures ofthe oxygen in a pipe in which said membrane is located and the oxygen inthe atmosphere, the effective area of the permeating membrane, thetemperature of the atmosphere or the flow rate of the gas in the pipe.6. The method for measuring a trace quantity of oxygen in a gasaccording to claim 1, wherein the constant amount of oxygen to besupplied is supplied to a carrier gas for oxygen through an oxygenpermeating membrane.
 7. The method for measuring a trace amount ofoxygen in a gas according to claim 6, wherein the amount of the constantamount of oxygen to be continuously supplied is controlled bycontrolling the temperature in a thermostatic bath harboring the oxygenpermeating membrane.
 8. In a method for measuring a trace quantity ofoxygen in a gas comprising reacting yellow phosphorus vapor with theoxygen in a sample of said gas to provide light and measuring thequantity of the oxygen in the sample of said gas based on the measuredintensity of the light emitted by the reaction, said measured intensityhaving a non-linear detector response in the range of 0-100parts-per-billion wherein the improvement comprises continuouslysupplying a constant amount of oxygen together with the sample of thegas, the amount of oxygen continuously supplied being effective to shiftthe intensity of the light emitted by the reaction to a range in whichthe non-linear detector response is substantially removed wherein themeasured intensity is the light emitted by the reaction of the yellowphosphorus vapor and both the oxygen in the sample and the oxygencontinuously supplied with the sample and determining an oxygenconcentration from the difference between the measured amount of lightand an intensity emitted by a reaction between the yellow phosphorusvapor and the oxygen continuously supplied.
 9. The method for measuringa trace quantity of oxygen in a gas according to claim 8, wherein thelight emitted by the reaction between the yellow phosphorus vapor andboth the oxygen continuously supplied and the oxygen in the sample ismeasured by a photodetector and the oxygen concentration is determinedfrom the output of the photodetector.
 10. The method for measuring atrace quantity of oxygen in a gas according to claim 8, wherein theconstant amount of oxygen is continuously supplied by supplying anoxygen-containing gas containing the constant amount of oxygen.
 11. Themethod for measuring a trace quantity of oxygen in a gas according toclaim 8, wherein the constant amount of oxygen to be continuouslysupplied is supplied to the sample gas or to a carrier gas for yellowphosphorus vapor through an oxygen permeating membrane located in a pipeby using the difference in the partial pressures of the oxygen in thepipe and the oxygen in the atmosphere, the effective area of thepermeating membrane, the temperature of the atmosphere or the flow rateof the gas in the pipe.
 12. The method for measuring a trace quantity ofoxygen in a gas according to claim 8, wherein the constant amount ofoxygen to be supplied is supplied to a carrier gas for oxygen through anoxygen permeating membrane.
 13. The method for measuring a tracequantity of oxygen in a gas according to claim 8, wherein the amount ofoxygen continuously supplied together with the sample is 30 ppb or less.14. In a method for measuring a trace quantity of oxygen in a gascomprising reacting yellow phosphorus vapor with the oxygen in a sampleof said gas to provide light and measuring the quantity of the oxygen inthe sample of said gas based on the measured intensity of the lightemitted by the reaction, said measured intensity having a non-lineardetector response in the range of 0-100 parts-per-billion wherein theimprovement comprises continuously supplying a constant amount of oxygentogether with the sample of the gas, the amount of oxygen continuouslysupplied being effective to shift the intensity of the light emitted bythe reaction to a range in which the non-linear detector response issubstantially removed wherein the measured intensity is the lightemitted by the reaction of the yellow phosphorus vapor and both theoxygen in the sample and the oxygen continuously supplied with thesample;measuring said shifted intensity; calculating an oxygenconcentration from said measured intensity of light; and calculating adifference between said oxygen concentration and the concentration ofthe continuously supplied oxygen.
 15. The method for measuring a tracequantity of oxygen in a gas according to claim 14, wherein the lightemitted by the reaction between the yellow phosphorus vapor and both theoxygen continuously supplied and the oxygen in the sample is measured bya photodetector and the oxygen concentration is determined from theoutput of the photodetector.
 16. The method for measuring a tracequantity of oxygen in a gas according to claim 14, wherein the constantamount of oxygen is continuously supplied by supplying anoxygen-containing gas containing the constant amount of oxygen.
 17. Themethod for measuring a trace quantity of oxygen in a gas according toclaim 14, wherein the constant amount of oxygen to be continuouslysupplied is supplied to the sample gas or to a carrier gas for yellowphosphorus vapor through an oxygen permeating membrane located in a pipeby using the difference in the partial pressures of the oxygen in thepipe and the oxygen in the atmosphere, the effective area of thepermeating membrane, the temperature of the atmosphere or the flow rateof the gas in the pipe.
 18. The method for measuring trace quantity ofoxygen in a gas according to claim 14, characterized in that theconstant amount of oxygen to be supplied is supplies to a carrier gasfor oxygen through an oxygen permeating membrane.
 19. The method formeasuring a trace quantity of oxygen in a gas according to claim 14,wherein the amount of oxygen continuously supplied together with thesample is 30 ppb or less.